Don't go in full day light.
Take a water bottle, and a stroller if you're not a purist about the children walking the distance.
We learned that the planets are really really spaced out. We kept track of the number of steps.
And we didn't make it all the way to Pluto.
Somewhere about Saturn M was on the verge of heat stroke (probably not, but for her to be red in the cheeks, it takes a lot). So we sat down at the base of the marker and Dad went back for the car.
I had the idea that we'd rest then continue to make progress. So we rested up a bit and then kept walking. And we had to keep stopping. Poor kids. We had just barely reached 1849 steps (13 past Neptune) past when P pulled up on the curb and put an end to the grand adventure.